Rotary actuators configured from a flat strain wave gearing reduction drive and a motor are known in the art. A flat strain wave gearing reduction drive comprises a cylindrical flexible externally toothed gear. One axial-direction side of the flexible externally toothed gear meshes with a stationary-side rigid internally toothed gear having a different number of teeth, and the other axial-direction side meshes with a drive-side rigid internally toothed gear having the same number of teeth as the flexible externally toothed gear. Thrust acts on the flexible externally toothed gear when the flat strain wave gearing reduction drive is in operation, and the flexible externally toothed gear moves in the axial direction. Therefore, it is necessary to arrange a regulating member for regulating axial-direction movement of the flexible externally toothed gear.
Patent Document 1 proposes a steering device incorporating a rotary actuator that comprises a flat strain wave gearing reduction drive. In Patent Document 1, a protective plate that is press-fitted and secured to a stationary-side rigid internally toothed gear is arranged between a motor housing and a flexible externally toothed gear that are mutually opposed along the axial direction. The protective plate prevents the flexible externally toothed gear from moving in the axial direction and interfering with the motor housing.